Conventionally, the electronics industry has used screen or stencil printing for mass production soldering of Integrated Circuits (ICs) to Printed Circuit Boards (PCBs). This process involves placing apertures in a metal plate, or stencil, where solder is desired on the PCB. Solder paste is placed on the stencil and pushed through the apertures by a squeegee that moves the solder paste over the stencil. The ICs' leads are then placed in the solder paste deposits on the PCB. Next the PCB, with the ICs, is placed in an oven and heated to at least a liquidous temperature, where the solder particles are melted. This forms a connection between the ICs and the PCB.
As ICs have become more complex, by packaging more functions in a smaller area, the number of leads has increased and the distance between the leads (pitch) has decreased. In fine pitch (i.e., closely spaced leads) applications the importance of printing consistent solder paste volume and alignment is critical. Excessive paste or misalignment results in shorts or bridges and too little paste results in poor connections. These problems show up as defects in the product, resulting in quality problems costing time and money in rework and warranty repairs. This problem is particularly acute in surface mount technologies that have very fine pitches.
The electronic industry has invested a great deal of time and money trying to understand and characterize the screen printing process, in the hope of discovering the key to obtaining consistent solder paste volumes. Research shows that paste volumes are effected by the variance in the apertures in the stencil. Variances in paste volume are also correlated to solder particle size, the speed at which the stencil is removed from the PCB after printing, aperture shape and a wide variety of other factors. Unfortunately, the research also shows that even after controlling for all these factors significant variance in solder paste volumes still occur in fine pitch applications (i.e., 0.025 inches or 0.635 mm).
The electronic industry has developed sophisticated optical alignment techniques to insure the stencil is correctly aligned with the PCB. Despite these precise alignment techniques solder paste deposits are sometimes misaligned with the pads on the PCB.
Thus, there exists a need for a stencil printing method and apparatus that can reduce the variance in solder paste volumes and decrease the occurrences of misaligned solder paste deposits.